Electrochemical fuel cells employing membrane electrode assemblies are known and have been produced and sold for many years. Such cells are known as solid polymer type fuel cells and comprise, in the heart of the system, two porous electrodes separated by an electrolyte material in the form of a membrane. The porous electrodes, conveniently made from carbon fiber paper ("CFP") supporting a layer of a catalyst such as platinum, and the electrolyte material together form an assembly called a membrane electrode assembly ("MEA"). The MEA is located between two electrically conductive or, conveniently, graphite flow field plates. The graphite flow field plates supply fuel and oxidant typically in the form of hydrogen and air or oxygen, respectively, to the MEA and also act to transmit current generated by the fuel cell stack to an external electrical circuit where it may be stored or otherwise used. The fuel and oxidant are supplied to the MEA by grooves in the surface of the graphite flow field plates adjacent the carbon fiber paper. The grooves communicate with manifolds carrying gases to each of the individual MEAs.
The membrane electrode assembly includes a catalytic material, conveniently platinum as previously stated, on the surface of the CFP which renders that portion of the CFP an electrode. The electrode portion of the CFP contacts the membrane. The CFP is made hydrophobic, typically by the incorporation of polytetrafluoroethylene (tradename Teflon). Ridges between the grooves in the graphite flow field plates contact the back of the electrode portion of the CFP. The MEA consumes the fuel and oxidant through an electrochemical process and produces an electrical current which can be drawn from the electrodes to an external circuit.
To ensure that the fuel and oxidant gases supplied to the MEA do not mix, sealing to prevent such mixing is imperative. If the hydrogen and oxygen combine within the fuel cell in combination with the Catalyst, a combustible mixture can form and inflame. If the fuel and oxidant leak from the interior to the exterior of the fuel cell, the efficiency of the fuel cell can be reduced and a fire or explosive hazard created.
In conventional fuel cells, an MEA was interposed between the two electrically conductive, preferably graphite, plates. The MEA comprised a membrane bonded between the CFP layers. The membrane extended substantially beyond the edge or periphery of the CFP layers and that outer portion of the membrane was not supported by or bonded to them. The CFP layers covered only the inner or active portion of the membrane. The outer portion or periphery of the membrane was free of the CFP. The periphery of the membrane was installed between two adjacent electrically conductive plates and acted as a gasket, sealing the gases in the electrode region from the exterior, isolating the gases in their respective manifolds, and electrically insulating the electrical conducting flow field plates between which the membrane was installed.
This conventional membrane electrode assembly was disadvantageous in several respects. First, the membrane did not function well as a gasket. The membrane was subject to shrinking and swelling depending on the water content. Since it was free to shrink and swell, its potential for tearing or for developing fatigue cracks was high. Although various techniques were utilized in an attempt to minimize the leaks across the membrane between the flow field plates, the techniques were expensive and substantially ineffective over an extended time period.
Furthermore, using the unsupported outer edges of the membrane to serve as an insulator and gasket between the opposing flow field plates placed strength and resilience demands upon the membrane which limited the minimum thickness of the membrane which could realistically be used in a fuel cell. An unsupported membrane having an inadequate thickness will be prone to failure due to its expansion and contraction in operation. Such thin membranes are subject to tearing when the cell is assembled or disassembled and when the membrane electrolyte is cycled between the hydrated operating state and the dehydrated non-operating state.
Up to a point, it is desirable to reduce the thickness of the membrane electrolyte since the electrolyte represents a substantial component of the internal electrical resistance of the fuel cell. A fuel cell with a thinner electrolyte will have a lower internal resistance and thus a higher voltage will be available at the fuel cell terminals for a given current demand. This translates directly into a greater power and fuel efficiency being derived from a fuel cell with a thinner electrolyte. The advantage of greater power and fuel efficiency derived from a thinner membrane electrode is tempered only by the requirement that the membrane be sufficiently thick to sustain the operating pressure differential between the fuel and oxidant gases and to minimize the diffusional mixing of these two gas streams through the membrane.
In the conventional MEA it was also necessary to machine a recess in each flow field plate contiguous with the periphery of the CFP layers and approximately the thickness of the CFP layers. By providing such a recess, the MEA could be positioned between the flow field plates while maintaining a uniform distance about the periphery of the flow field plates. Maintaining this uniform distance allowed the membrane to be tightened appropriately between the plates so as to provide a good sealing action. However, machining such a recess was time consuming, expensive and, in fact, did not assist substantially in enhancing the sealing action.
A further disadvantage in the conventional MEA was that the membrane itself was difficult to position and maintain in position while the fuel cell stack was being assembled. This difficulty was a result of the thinness and inherent inflexibility of the membrane. This difficulty was also the result of the tendency of the membrane to expand and contract due to the humidity changes in the gases to which the membrane was subjected.
One proposed method of sealing the MEA and the adjacent electrically conductive plate without using the membrane as a gasket is to form grooves in the surfaces of the electrodes facing away from the membrane and depositing sealant material into the grooves. This proposed sealing method presents several disadvantages. First, it has proven difficult to provide a uniform thickness of sealant material necessary for an optimal seal. Second, the sealant tends to deform in a nonuniform manner when compressed in the assembled fuel cell stack. Third, the extrudable sealant material is not sufficiently resilient to withstand compressive forces over time, and the extrudable sealant eventually deteriorates. This deterioration tends to worsen at elevated temperatures, such as those generated during fuel cell operation. The extrudable sealant material also tends to undergo chemical degradation when exposed to oxidants such as those found in fuel cells. Moreover, the use of extrudable sealant material required the machining of grooves to carry the sealant in the electrode sheet material. The machining of such grooves into the electrode portion of the membrane electrode assembly oftentimes damaged the membrane, and was also a time consuming and labor intensive task.